Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature properties of the engine components must correspondingly increase. While significant advances have been achieved through formulation of iron, nickel and cobalt-base superalloys, the high temperature properties of such alloys are often insufficient to withstand long exposures to operating temperatures within the turbine, combustor and augmentor sections of some high-performance gas turbine engines. As a result, internal cooling of components such as turbine blades (buckets) and nozzles (vanes) is generally necessary, and is often employed in combination with a thermal barrier coating (TBC) system that thermally protects their exterior surfaces. Effective internal cooling of turbine blades and nozzles often requires a complex cooling scheme in which bleed air is forced through serpentine passages within the airfoil and then discharged through carefully configured cooling holes at the airfoil trailing edge, and frequently also film cooling holes at the airfoil leading edge and/or cooling holes at the blade tip.
The performance of a turbine airfoil is directly related to the ability to provide a generally uniform surface temperature with a limited amount of cooling air. To promote convective cooling of the airfoil interior, it is conventional to cast turbulators, such as ribs or other surface features, in the interior surfaces that define the cooling passages. With film cooling holes, the size, shape and placement of the turbulators determine the amount and distribution of air flow through the airfoil cooling circuit and across the external surfaces of the airfoil downstream of the film cooling holes, and as such can be effective in significantly reducing the service temperature of the airfoil. Turbulators are typically employed throughout the interior cooling passages of an airfoil in order to promote cooling. To maximize heat transfer efficiency, turbulators are often formed on the interior surfaces of the airfoil sidewalls, often termed the pressure and suction walls, the former of which has a generally concave exterior profile while the latter has a generally convex exterior profile.
While cooling circuits, cooling holes and turbulators have been developed that significantly increase the maximum operating temperatures sustainable by turbomachinery airfoils, further improvements would be desirable in order to further extend airfoil life and increase engine efficiency.